Method and apparatus for adding a communication connection to a vectored group

ABSTRACT

Methods for adding a communication connection to a vectored group of communication connections and corresponding apparatuses are disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. application Ser. No.12/109,109 filed on Apr. 24, 2008, which is incorporated herein byreference in its entirety.

BACKGROUND

So-called vectoring or vectored data transmission is a technique forcoordinated transmission or reception of data from a plurality oftransmitters to a plurality of receivers via a plurality ofcommunication connections in order to improve the transmission, forexample to reduce the influence of crosstalk. Either transmitters orreceivers are co-located.

For example, in DSL (digital subscriber line) transmission systems, forexample VDSL (very high bit rate DSL) transmission systems, data may betransmitted from a central office (CO) or other provider equipment to aplurality of receivers located in different locations, for example incustomer premises (CPE), via a plurality of communication lines.Crosstalk resulting from signals on different lines transmitted in thesame direction, also referred to as far end crosstalk (FEXT), may resultin a reduced data throughput. Through vectoring, signals transmittedover the plurality of communication lines from the central office orreceived via the plurality of communication lines in the central officemay be processed jointly in order to reduce such crosstalk, which jointprocessing corresponds to the above-mentioned vectoring. In thisrespect, the reduction of crosstalk by coordinated transmission ofsignals is sometimes referred to as crosstalk precompensation, whereasthe reduction of crosstalk through joint processing of the receivedsignals is sometimes referred to as crosstalk cancellation. Thecommunication connections which are processed jointly are sometimesreferred to as vectored group.

Both at the initialization of communication and during communication, itmay be necessary to add an additional communication connection to thevectored group, for example when an additional user of a DSL servicebecomes active. In such a case, it is generally desirable to minimizethe influence of this joining of a new communication connection on thedata transmission occurring in the communication connections already inthe vectored group.

SUMMARY

In an embodiment, a method is provided, said method comprising:

a first adapting phase wherein vectoring is adapted to reduce crosstalkfrom a communication connection to be added to the vectored group to atleast one communication connection of the vectored group,

after said first adapting phase, an initialization phase wherein datatransmission on said communication connection to be added to thevectored group is at least partially initialized, and

after said initialization phase, a second adapting phase wherein thevectoring is adapted to reduce crosstalk from at least one communicationconnection of the vectored group to the communication connection to beadded to the vectored group.

The above summary is merely intended to give a brief overview of somefeatures of some embodiments of the present invention, and otherembodiments may comprise additional and/or different features than theones mentioned above. In particular, this summary is not construed to belimiting the scope of the present application.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 shows a communication system according to an embodiment of thepresent invention,

FIG. 2 shows a flow diagram of a method according to an embodiment ofthe present invention,

FIG. 3 shows a timing diagram of an embodiment of the present invention,

FIG. 4 shows a block diagram of a communication system according to anembodiment of the present invention,

FIG. 5 shows a block diagram of the communication system of FIG. 4operated in a different transmission direction,

FIG. 6 schematically depicts data transmission according to anembodiment of the present invention, and

FIG. 7 schematically depicts data transmission according to anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following, some embodiments of the present invention will bedescribed in detail. It is to be understood that the followingdescription is given only for the purpose of illustration and is not tobe taken in a limiting sense. The scope of the invention is not intendedto be limited by the embodiments described hereinafter with reference tothe accompanying drawings, but is to be intended only to be limited bythe appended claims and equivalents thereof.

It is also to be understood that in the following description ofembodiments any direct connection or coupling between functional blocks,devices, components, circuit elements or other physical or functionalunits shown in the drawings or described herein could also beimplemented by an indirect connection or coupling. Furthermore, itshould appreciated that functional blocks or units shown in the drawingsmay be implemented as separate circuits in embodiments, but may also befully or partially implemented in a common circuit in other embodiments.It is further to be understood that any connection which is described asbeing wire-based in the following specification may also be implementedas a wireless communication unless noted to the contrary.

It should be noted that the drawings are provided to give anillustration of some aspects of embodiments of the present invention andtherefore are to be regarded as schematic only. In particular, theelements shown in the drawings are not necessary to scale with eachother, and the placement of various elements in the drawings is chosento provide a clear understanding of the respective embodiment and is notto be construed as necessarily being a representation of the actualrelative locations of the various components in implementationsaccording to an embodiment of the invention.

The features of the various embodiments described herein may be combinedwith each other unless specifically noted otherwise.

The term “communication connection” as used herein is intended to referto any kind of communication connection including wire-basedcommunication connections and wireless communication connections.

In FIG. 1, a communication system according to an embodiment of thepresent invention is schematically shown.

In the communication system of FIG. 1, a communication device 10communicates with communication devices 16, 17, 18 and 19 via respectivecommunication connections 12, 13, 14 and 15. While in FIG. 1 fourcommunication devices 16, 17, 18 and 19 are shown, in other embodimentsany suitable other number of communication devices may also be provided.

In an embodiment, the communication via communication connections 12,13, 14 and 15 is a bidirectional communication. In such an embodiment,communication device 10 may comprise a transceiver for each of thecommunication connections 12, 13, 14 and 15, and each communicationdevice 16, 17, 18 and 19 also may comprise a transceiver. In anotherembodiment, all or some of communication connections 12, 13, 14 and 15may be unidirectional communication connections. In another embodiment,all or some of the communication devices 16, 17, 18, 19 might beco-located.

In the embodiment of FIG. 1, couplings between the communicationconnections 12-15 may cause crosstalk, for example if some or all of thecommunication connections are wire lines running close to each other.Through at least partial joint processing of the signals transmittedfrom communication device 10 to communication device 16, 17, 18 and 19and through at least partial joint processing of signals received fromcommunication devices 16, 17, 18 and 19 at communication device 10 in acrosstalk reduction unit 11, the influence of such crosstalk may bereduced. As already mentioned, the joint processing for crosstalkreduction is also referred to as vectoring, and the communicationconnections which are subjected to such a crosstalk reduction are alsoreferred to as vectored group.

In the following, the transmission direction from communication device10 to communication devices 16, 17, 18 and 19 will be referred to asdownstream direction, and the opposite transmission direction fromcommunication devices 16, 17, 18 and 19 to communication device 10 willbe referred to as upstream direction. Reduction of crosstalk in thedownstream direction is also referred to as crosstalk precompensationsince the signals transmitted are modified before transmission, i.e.before the actual crosstalk occurs, whereas the reduction of crosstalkin the upstream direction is also referred to as crosstalk cancellationas here through joint processing in crosstalk reduction unit 11 thecrosstalk is reduced or cancelled after it has occurred.

In embodiments, crosstalk cancellation may for example be performed bycalculating received signals for each communication connection dependingon a linear combination of all received signals on all communicationconnections of the vectored group, and crosstalk precompensation may beperformed by calculating signals to be transmitted via eachcommunication connection depending on a linear combination of signals tobe transmitted on all communication connections. However, othercalculation methods, for example non-linear calculations, are alsopossible.

In order to perform this crosstalk reduction, i.e. the vectoring, thecrosstalk reduction unit 11 has to be “trained”, i.e. the crosstalkreduction unit 11 needs information regarding the actual crosstalkoccurring between the communication connections in the vectored group.This may for example be achieved by transmitting predetermined trainingsignals, for example pilot signals, via the communication connectionsand analyzing the received signals to determine the crosstalk. Inembodiments, data transmission via the communication connectionscomprises the transmission of pilot signals or symbols, wherein betweenthe pilot signals other data like payload data may be transmitted. In anembodiment, the pilot signals or modified pilot signals are used fortraining crosstalk reduction unit 11. In an embodiment, synchronizationsignals or synchronization symbols may be used as pilot signals.

In a communication system like the one shown in FIG. 1, the situationmay occur that a communication connection is to be added to the vectoredgroup. For example, in the embodiment of FIG. 1 initially onlycommunication connections 12, 13 and 14 may be included in the vectoredgroup, while communication connection 15 may be inactive (for examplecommunication device 19 may be switched off) and therefore not be addedto the vectored group. When communication device 19 becomes active, inorder to also reduce the crosstalk between communication connection 15and communication connections 12-14 which already are incorporated inthe vectored group, communication connection 15 is to be added to thevectored group. For such an additional communication connection to beadded, crosstalk reduction unit 11 has to be trained and adaptedaccordingly.

In an embodiment, for adding an additional communication connection(like communication connection 15 in the above example) to a vectoredgroup, a first vector training is performed to reduce or cancel thecrosstalk resulting from the influence of the communication connectionto be added, hereinafter also referred to as the joining connection, tothe communication connections already in the vectored group. When thisfirst vector training is completed, the influence of data transmittedover the joining connection on the data transmission of the connectionsalready in the vectored group is minimized or at least reduced by anadaption of the crosstalk reduction coefficients of the connectionsalready in the vectored group in crosstalk reduction unit 11 of theembodiment of FIG. 1. In an embodiment, after this first training, thejoining communication connection is at least partially initialized,which may involve for example training steps like obtaining asynchronization, adapting coefficients of receivers in the involvedcommunication devices, for example coefficients for a frequencyequalization, adjusting the gain of an amplifier and other training ofparameters or exchange of configuration parameters. This initializationof the communication connection may in an embodiment partially overlapwith the above-mentioned first vector training. Training steps in theinitialization for the joining line may comprise a training forparameters related solely to the joining communication connection. Thetraining may also consist of or incorporate training sequences similaror identical to training sequences for non-vectorized transmissionstandards for example the channel discovery training sequence defined inADSL and VDSL. In general, in the context of the present applicationinitialization refers to any measure taken for establishing and settingup communication via a connection before transmission of payload datastarts. As mentioned above, after the first training the joiningcommunication connection is at least partially initialized, meaning thatnot all measures, steps and the like for initialization have to beperformed at this point, but parts of the initialization may beperformed earlier or later. In another embodiment, the completeinitialization of the joining line may be performed at this point.

After this at least partial initialization of the joining communicationconnection, in the currently discussed embodiment a second vectortraining is performed to adapt the vectoring to reduce or cancelcrosstalk resulting from the influence of the communication connectionsof the vectored group to the joining communication connection. With theorder of the training and initialization phases of the embodimentdescribed above, the at least partial initialization of the joiningcommunication connections does not influence data transmission of thecommunication connections already in the vectored group since thevectoring has been adapted in the first vector training to reduce thecrosstalk from the joining communication connection to the communicationconnections of the vectored group. On the other hand, the second vectortraining is performed after the above described at least partialinitialization of the joining communication connection allowing anymodifications to the data transmission on the joining communicationconnection performed during the initialization of the joiningcommunication connection to be taken into account in the second vectortraining. This may for example increase the data rate at which thejoining line can transmit during at the second vector training. In anembodiment, in the second vector training also the reduction ofcrosstalk from the joining connections to the communication connectionsof the vectored group may be re-trained.

In an embodiment, an apparatus like communication device 16, 17, 18 or19 may be configured to receive vector training signals during the firstvector training mentioned above. The apparatus may be further configuredto, during the at least partial initialization mentioned above receiveand/or transmit initialization signals and to adapt itself to establishcommunication according to the initialization signals, and configured totransmit vector training signals during the second vector trainingmentioned above.

In one embodiment, which may be combined with the aforementionedembodiments, but also used separately, the vector training is generallyperformed using modified non-payload data carrying signals like pilotsymbols or pilot signals as mentioned above. For example,synchronization symbols used in a standardized communication method likeVDSL to synchronize super frames may be multiplied with sequences of +1and −1, the sequences of the different channels being chosen such thatthey are orthogonal to form pilot symbols. For example, Walsh-Hadamardsequences may be used as the modulation sequence.

In such an embodiment, a first vector training to reduce crosstalk fromthe joining communication connection to the communication connections inthe vectored group in the downstream direction is performed, whereinbetween modulated synchronization signals so-called quiet signals, i.e.when no power or signals with very low transmission power, or signalswhere only one or more pilot carriers are active (also referred to ascarrier-pilot signals), are transmitted. The modulated synchronizationsignals in the downstream direction are synchronized withsynchronization signals transmitted in the communication connectionsalready in the vectored group in the downstream direction. By doing sothe transmission of the modulated synchronization signals which are usedfor the vector training of the cross coupling coefficients does notdisturb the actual data transmission on the communication connections inthe vectored group since the data transmission on the communicationconnections in the vector takes place in the time slots between thesynchronization signals and since the joining line is synchronized fordownstream transmission with the vectored group. The joining line willtransmit in these time slots the quiet signals. Therefore, the system isallowed to start right at the beginning of the joining procedure withoutthe need for waiting for initialization steps such as training orexchange of configuration parameters of the joining line orsynchronization at the far end device 19 of the joining line since inthe vector training in the downstream direction the training signals onthe joining line and the data communication signals on the existing linealready in the vector group are transmitted from a same device i.e. thecommunication device 10 which is able to establish synchronization indownstream direction with the communication connections already in thevector group. At this point, the communication device receiving themodulated synchronization signals used for the vector training, in theexample given above communication device 19 receiving the signals viacommunication connection 15 from communication device 10, has notobtained the synchronization timing of transmitted synchronizationsignals. After the vector training of the communication connectionsalready in the vector group in the downstream direction using thesynchronization signals transmitted described above, further trainingsignals are transmitted in the downstream direction allowing thereceiving communication device to recover, i.e. obtain, the timing ofthe synchronization signals. After the timing has been recovered, thecommunication device may then in an embodiment transmit synchronizationsignals in the upstream direction synchronized with the synchronizationsignals sent in the upstream direction on the communication connectionsalready in the vectored group for vector training in the upstreamdirection.

In another embodiment, the synchronization of the communication devicereceiving signals in the downstream direction may be obtained at leastpartially based on the synchronization signals sent during theabove-mentioned downstream vector training phase.

The above-mentioned downstream- and upstream training phases may forexample be used in the first vector training step described in thepreviously discussed embodiment.

It should be noted that the embodiments for adding a communicationconnection to a vectored group of communication connections describedherein may be used both as startup of the whole system for adding onecommunication connection after the other to the vectored group and, whenafter the initial set up of the system an additional communicationconnection is to be added to the vectored group.

In the following, further embodiments of the present invention will bedescribed with reference to FIGS. 2-7 using VDSL2 transmission as anexample. VDSL2 is a standard described in ITU-T recommendation G.993.2.However, it should be noted that the embodiments described in thefollowing may also be applied to other types of communication than VDSL2communication, like other types of DSL communication or wirelesscommunication.

In an embodiment using VDSL2 communication, a communication systemcorresponding to the one already described with reference to FIG. 1 maybe used. In this case, communication connections 12-15 are VDSLcommunication lines, and the communication devices are VDSLcommunication devices. In such a system, communication device 10 may bepart of so-called central office (CO) equipment, for example equipmentof a provider of the VDSL service, whereas communication devices 16-19may be located in customer premises (CPE).

In FIG. 2, a flow diagram of a method for adding a communication line(referred to as joining line hereinafter) to a vectored group ofcommunication lines e.g. in a VDSL2 system according to an embodiment ofthe present invention is shown.

In phase 20 of the embodiment of FIG. 2 a so-called handshake (ITU-Trecommendation G. 994.1) is performed on the joining communication line.The handshake is a standardized procedure when a communication linebecomes active wherein for example a mode of operation of the equipmentused is established and the handshake uses only a very reduced number ofsubcarriers resulting avoiding influences on the existing communicationlines.

In phase 21, a first vector training is performed. In this first vectortraining, the effect of crosstalk from the joining line to the linesalready in the vectored group is reduced. Taking the system of FIG. 1 asan example, when it is assumed then that the communication lines 12-14are already in the vectored group and communication line 15 becomesactive and is to be added to the vectored group, in the first vectortraining of phase 21 the effects of crosstalk from communication line 15to communication lines 12-14 is reduced by adapting the operation ofcrosstalk reduction unit 11 accordingly.

After phase 21, in phase 22 a channel discovery for the joiningcommunication line is performed, and in phase 23 a training is performedfor further adapting various parameters in the receivers receiving datatransmission via the communication line to be added. In the examplegiven above with respect to FIG. 1, for example the receiver incommunication device 19 and the receiver in communication device 10coupled to communication line 15 are trained. In VDSL2 datatransmission, channel discovery and training are standardized steps ofan initialization phase. In other communication modes, similar steps maybe standardized. Since in the first vector training step 21 of thisembodiment the crosstalk from the joining line to the lines already inthe vectored group has been reduced, phases 22 and 23 which involvetransmitting signals including data over the joining communication linedo not or do not significantly influence data transmission on thecommunication lines already in the vectored group.

It should be noted that in an embodiment phases 21 and 22 may be atleast partially combined, i.e. the channel discovery may be performedfully or partially during or interleaved with the first vector training.According to one embodiment, the first vector training is timely splitfor the upstream and downstream transmission such that the downstream isperformed prior to the upstream training. In such an embodiment, phase22 may already be started for the downstream direction while theupstream direction is still performing phase 21. The above allows topartially combine the channel discovery of existing standards into avectored system while allowing a start of the vector training at a veryearly point of time in the joining procedure.

In an embodiment, in phases 22 and 23 a so-called back channel isestablished in the joining communication line over which in response tosignals in the downstream direction messages may be sent back to thecommunication device transmitting the signals in the downstreamdirection, e.g. to the central office, for example to give informationabout receiving errors and the like. Taking again FIG. 1 as an example,such a back channel may be used to transmit status messages, informationmessages and the like from communication device 19 to communicationdevice 10.

Following phase 23, in phase 24 a second vector training is performed,in which the vectoring is adapted to reduce crosstalk from thecommunication lines already in the vectored group to the joiningcommunication line. In an embodiment, the second vector training mayalso involve adjusting the crosstalk related parameters determinedduring the first vector training to adapt to changes in thecommunication over the joining line caused by phases 22 and 23.

In phase 25, a so-called channel analysis and exchange is performed,which again is a step standardized for example in VDSL communication.

Finally, in phase 26 so-called “showtime” begins, i.e. the joiningcommunication line now has joined the vectored group and is used fortransmission of payload data.

Phases 22, 23 and 25 according to VDSL2 are parts of the so-calledinitialization of the joining communication line. In the embodiment ofFIG. 2 therefore the initialization is at least partially performedbetween the first vector training of phase 21 and the second vectortraining of phase 24, another part of the initialization, namely thechannel analysis and exchange, being performed after the second vectortraining of phase 24. In another embodiment, the complete initializationmay be performed between the first vector training of phase 21 and thesecond vector training of phase 24.

In an embodiment, for the first vector training and the second vectortraining mentioned above so-called synchronization symbols (SyncSymbols)of VDSL2 are used as pilot symbols. For example, in VDSL2 system, asynchronization symbol is sent every 257^(th) symbol. Thesesynchronization symbols may be modified to form sequences ofsynchronization symbols for each time, wherein the sequences in each ofthe communication lines involved are orthogonal to each other. Forexample, the Sync-Symbols may be multiplied with predetermined series of−1 and +1, the sequences corresponding to Walsh-Hadamard sequences of apredetermined length, for example a length of 64 symbols. However, othertypes of orthogonal or almost orthogonal sequences may be used as wellin an embodiment.

In the following, the first vector training and the second vectortraining according to embodiments of the present invention will bediscussed in greater detail.

In this respect, FIG. 3 shows a detailed timing diagram of differentphases of signal transmission via a joining communication linecomprising signals transmitted during the first vector trainingaccording to an embodiment, FIGS. 4 and 5 show exemplary diagrams ofcommunication systems during training in the downstream direction andtraining in the upstream direction, respectively, and FIG. 6 shows adiagram showing signals transmitted for recovering a synchronizationtiming at a receiver.

In FIG. 3, the phases of signal transmission are schematically orderedin four columns. The two left columns relate to signals and messagestransmitted in the downstream direction, and the two columns to theright relate to signals and messages transmitted in the upstreamdirection. In each transmission direction, a discrimination is madebetween messages sent on the so-called special operations channel (SOC)which is specified according to VDSL2 standard (shown in the leftmostand rightmost column) and other signals transmitted (shown in the twomiddle columns). For these special operations channels, an HDLC (HighLevel Data Link Control) like protocol may be used.

In an embodiment, the timing diagram of FIG. 3 represents a modificationof the conventional timing diagram of a part of the channel discoveryphase as defined in the VDSL2 standard G.993.2.

The data transmission both in the downstream direction and in theupstream direction starts with the signals necessary for the handshake,denoted with reference numeral 30 in the downstream direction and 40 inthe upstream direction. In the downstream direction, after the signalsnecessary for the handshake have been transmitted, at 31 a phase whereso-called quiet symbols are transmitted follows denoted with referencenumeral 31. Quiet symbols are signals having no or very little transmitpower, for example generated by modulating only zeros on carriers usedfor VDSL transmission. After phase 31, a phase 32 labeled O-P-Vectoring1 is initiated. During this phase, the crosstalk from the joiningcommunication line to the communication lines already in the vectoredgroup in the downstream direction is reduced. It should be noted thatphase 31 in a different embodiment may also be omitted, such that phase32 immediately follows the handshake signals sent in phase 30. Phase 32,i.e. the phase labeled O-P-Vectoring 1, will be explained in more detailwith reference to FIG. 4.

In FIG. 4, a communication system according to an embodiment of thepresent invention is shown transmitting data in the downstreamdirection. In the system shown in FIG. 4, data is transmitted from acentral office 69 via a plurality of communication lines 55, 56, 57 to aplurality of receivers in customer premises generally labeled 84. In thesystem of FIG. 4, the communication lines are joined in a so-calledcable binder 58. Communication lines in a cable binder are usuallylocated comparatively close to each other and are therefore prone tocrosstalk. In the system shown in FIG. 4, communication lines 56 and 57as well as further (not shown) communication line indicated by dottedlines are already incorporated in the vectored group. It should be notedthat the number of communication lines in the vectored group is notlimited to any particular number. Communication line 55 in the exampleshown is the joining line, i.e. is to be added to the vectored group.

In the system of FIG. 4, a symbol mapper denoted with reference numerals70, 71 and 72 maps data, e.g. payload or training data, onto carrierconstellations which are to be transmitted via communication lines 55,56 and 57, respectively. A crosstalk precompensator 73 modifies thesesymbol mappings in order to precompensate crosstalk occurring during thetransmission. The such modified carrier mappings are modulated onto aplurality of carriers for each communication line, said carriers havingdifferent frequencies, and are then transferred into signals in the timedomain by inverse fast Fourier transformations 74, 75 and 76,respectively. This type of modulation, also referred to as discretemultitone modulation (DMT) is commonly used in DSL systems like VDSLsystems or VDSL2 systems. The such generated signals are thentransmitted via the communication line to the customer premises. Thereceived signals are then converted into the frequency domain by fastFourier transformers 77 and 80, respectively and equalized by frequencyequalizers 78, 81, respectively before slicers 79 and 82, respectivelyoutput received constellations which, in case of an error-freetransmission, correspond to input constellations generated in 71, 72originally intended for transmission. It should be noted that also acustomer premises equipment of the joining lines may have elementscorresponding to the elements shown for the vectored lines like fastFourier transformer, frequency equalizer and slicer, but these are notshown in FIG. 4. Moreover, it is to be understood that for clarity'ssake only some elements of the communication devices involved are shown,and further devices like amplifiers, sampling units and the like may bepresent.

During the O-P-Vectoring 1 of phase 32 of FIG. 3, crosstalk from thejoining line 55 to the lines already in the vectored group like lines 56and 57 is to be reduced, the crosstalk being indicated in FIG. 4 bydashed arrows in crosstalk precompensator 73.

As already mentioned, for adapting the vectoring which is implemented inthis case by crosstalk precompensator 73 to the joining line,synchronization symbols transmitted are modified to form orthogonalpilot sequences. The synchronization symbols on all the lines are sentin a synchronized manner, i.e. at the same time. Since in the downstreamdirection these signals are sent by central office 69, at this stage itis not necessary that the corresponding receiver connected to thejoining line in the customer premises has obtained this synchronization,i.e. it is not necessary that the receiver at this stage is alreadyadapted to the synchronization timing.

In an embodiment, a specific sequence is reserved for joining lines. Forexample, using the Walsh-Hadamard sequences multiplying thesynchronization symbols with sequences of +1 and −1 described above, asequence consisting only of +1 values or of alternating +1 and −1 valuesmay be reserved for joining lines.

On the joining line, in the embodiment of FIG. 3, during theO-P-Vectoring 1 phase quiet symbols or carrier-pilot symbols which onlyrelate to one or a few carriers are transmitted between thesynchronization symbols. Therefore, through crosstalk the datatransmission occurring in the vectored lines already in the vectoredgroup between the synchronization symbols is not or is not significantlydisturbed and can continue during this phase.

Via respective backchannels in the vectored lines, an error signal e istransmitted back to crosstalk precoder 73. Error signal e for thesynchronization symbols is indicative of a difference between thesynchronization symbols sent (which are known to the receiver sincefixed sequences are used) and the symbols actually received. Based onthis error signal, the crosstalk precompensator 73 is adapted to reducethe error, i.e. the difference between symbols sent and symbolsreceived, by reducing the effect of crosstalk from the joining line tothe vectored line. Therefore, after the phase O-P-Vectoring 1 of FIG. 3has been completed, the effect of crosstalk from the joining line to thevectored lines in the downstream direction can been reduced.

After this phase, signals may be transmitted on the joining line in thedownstream direction also in-between the synchronization symbols which,because of the crosstalk reduction performed, does not or does notsignificantly influence the data transmission in the downstreamdirection on the communication lines already in the vectored group in anembodiment of the present invention.

After phase 32, in the downstream direction symbols for a channeldiscovery, labeled O-P-channel discovery 1 in FIG. 3, are transmitted atreference numeral 33. Channel discovery itself is an initializationphase which is defined in various communication standards, for exampleVDSL2 communication. During this phase, channel discovery symbols aresent. In an embodiment, some of these symbols are modified to enable thereceiver which is in the system shown in FIG. 4 the customer premisesequipment connected to the joining line to obtain a timingsynchronization to the synchronization symbols sent, or, in other words,to adjust itself to the timing of the synchronization symbolstransmitted. A possible implementation according to an embodiment of thephase 33 will be explained with reference to FIG. 6.

In FIG. 6, the data transmission in downstream direction on the joiningline and on the vectored lines, i.e. the lines already in the vectorgroup, is schematically shown. On the vectored lines, VDSL2 data istransmitted in so-called superframes, each superframe comprising 257 DMTsymbols. Each superframe starts with a synchronization symbol 92followed by data symbols 93. With the data symbol, any kind of desireddata, in particular payload data, may be transmitted. In the embodimentof FIG. 6, the synchronization symbols as already discussed previouslyare modulated with predetermined sequences, for example withWalsh-Hadamard sequences of −1 and 1, such that the sequences in thedifferent vectored lines are orthogonal to each other. The sequences mayhave a predetermined length, for example 64 symbols. On the joiningline, symbols 90, 91 for channel discovery are transmitted in downstreamdirection, wherein the symbols on the joining line are at leastapproximately transmitted synchronously with the symbols on the vectoredlines. In order to enable the receiver at the subscriber end to recoverthe timing of the synchronization symbols, the symbols 90 which aresynchronously sent with the synchronization symbols 92 on the vectoredlines are marked such that they can be identified by the receiver.Marking may be a provided by using a modulation scheme different thanthe modulation scheme used for normal transmission operation. In anembodiment, to mark symbols 90 as being at the position of thesynchronization symbols 92, in an embodiment a modulation constellationat a specific carrier, at a particular group of carriers or at allcarriers are inverted. For example, in an embodiment the constellationsof carriers number 5, 15, 25, 35 . . . may be inverted.

In other embodiments, constellations of carriers numbered 10, 20, 30, .. . may be inverted or otherwise marked. In still other embodiments,other predetermined carriers may be used.

In another embodiment, every second symbol at a position of thesynchronization symbols may be marked, for example by invertingconstellations of carriers with pre-defined numbers inverted. Forexample every second symbol 90 in FIG. 6 may be marked. As an example,if carriers 10, 20, 30, . . . are used, the VDSL2 standard defines “00”as the “normal” constellation point to be used for these carriers of aO-P-CHANNEL DISCOVERY symbol. An inverted constellation point would thencorrespond to a “11” constellation.

Furthermore, in an embodiment not only the position of thesynchronization symbols is communicated in this manner, but it is alsocommunicated at which point sequences like the above-mentionedWalsh-Hadamard sequences start. In an embodiment, this is performed byrepeating, at the start of a new sequence, the constellation at theposition of the previous synchronization symbol on the predefinedcarriers. A simple example for this with a sequence length of 8 (forexample a Walsh-Hadamard sequence length of 8 symbols) will be explainedwith reference to the following table. However, it should be noted thatthe length of 8 only serves as an example, and other lengths, like theabove-mentioned length of 64 symbols, may be used as well.

TABLE 1 Example Sync Symbol Marker Sequence: Upstream Sync SymbolConstellation Point Sequence Index of Carriers 10, 20, 30 . . . . . . .. . 5 00 6 11 7 00 0 00 1 11 2 00 3 11 4 00 5 11 6 00 7 11 0 11 1 00 . .. . . .

In the left column of table 1, the sequence index of the synchronizationsymbols is shown, i.e. “0” denotes the first position of a sequence likea Walsh-Hadamard sequence and “7” denotes the eighth and in this examplethe last position of a sequence. In the right column, the constellationsof the carriers 10, 20, 30 . . . of the respective symbols, in thisexample training symbols, of the joining line at the position ofsynchronization symbols are shown, “00” as explained above for exampledenoting the “normal” constellation and “11” the inverted constellation.As can be seen, within a sequence, every second of such symbols isinverted, and at the beginning of a sequence (synchronization symbolsequence index 0) the previous constellation is repeated, such that twosuccessive equal constellations mark the beginning of the sequence. Inthis way, the receiver of the training symbols, in this example thereceiver at the subscriber end, can recover both the timing of thesynchronization symbols and the start of the sequence then used.

The above table shows merely one example for communicating thisinformation. In another embodiment, every training symbol at thelocation of the synchronization symbols may be marked with an invertedconstellation, for example “11”, at the predefined carriers used, forexample carriers 10, 20, 30, . . . , and the beginning of a sequence maybe marked with two or more successive symbols with predefined carriersinverted or otherwise marked, i.e. the symbol 91 immediately precedingthe training symbol 90 in FIG. 6 and/or the symbol 91 following trainingsymbol 90 may be inverted. In other embodiments, other predefinedsymbols before or after the position of the training symbol at thelocation of the synchronization symbols may be used for marking. Forexample, symbol 90 in FIG. 6 and the fourth and seventh following symbol91 may be inverted. Note that while constellation inversion is a simpleway of marking symbols, other type of constellation modification, suchas shifting or rotating, may also be used.

With the above-described signaling, for example the position of thesynchronization symbols in the upstream direction and the upstreamsequence start may be communicated to the subscriber. In anotherembodiment, additionally or alternatively, the locations of thesynchronization symbols in the downstream direction and/or the start ofthe downstream sequence may be signaled to the subscriber. Suchembodiments provide a robust way of communicating the position of thesynchronization symbols and the start of the training sequences.

In an embodiment, this inversion is performed during the completeinitialization process, e.g. up to the start of showtime. In a differentembodiment, the inversion is performed only during part of theinitialization process, for example only in phase 33 of FIG. 3(O-P-channel discovery 1). It should be noted that the above-describedembodiment of phase 33 for marking the positions of the symbols 90 maybe implemented independently from the remaining phases of FIG. 3.

In still another embodiment, the inversion, i.e. the marking of theposition of synchronization symbols is only performed in somepredetermined phases up to the start of showtime. As already mentioned,the scheme shown in FIG. 3 may be seen as a modification of the firstpart of the conventional channel discovery phase according to the VDSL2standard. Further phases defined in the VDSL2 standard apart from theabove-explained O-P-channel discovery 1 phase include the so-calledO-P-channel discovery 2 phase which is also part of the channeldiscovery according to VDSL2 or the so-called phases O-P-training 1 andO-P-training 2 which are part of the so-called training phase of theVDSL2 standard. For example, according to the VDSL2 standard theO-P-channel discovery 2 phase is used to send updated modulationparameters as well as information needed for the training phase such assignal durations. The symbols used during the O-P-channel discovery 2phase may be constructed in the same manner as the symbols used duringthe O-P-channel discovery 1 phase. The O-P-training 1 phase according toVDSL2 is used to resynchronize and establish correct symbol timing.During the O-P-training 2 phase according to VDSL2 the SOC messageexchange between the central office and the subscriber is reestablished.

In an embodiment, the inverted constellations or otherwise markedsymbols to communicate the position of the synchronization symbolsand/or the position of the start of sequences like the above-mentionedWalsh-Hadamard sequences are communicated during all four of theabove-mentioned phases, i.e. O-P-channel discovery 1, O-P-channeldiscovery 2, O-P-training 1 and O-P-training 2. In other embodiments,only two or three of these phases including O-P-channel discovery 1 areused. In other embodiments, other phases may be used. It should be notedthat in embodiments which are not based on the VDSL2 standard, phasesserving the same general purpose as the above-explained four phases maybe used for communicating synchronization symbol position and sequencestart position. Generally, embodiments, using more than one phase forcommunicating the above-explained information exhibit an increasedrobustness.

The above-described embodiments may be seen as examples forcommunication methods comprising:

-   transmitting in a vector training phase training signals from a    first communication device to a second communication device on a    communication connection to be added to a vectored group, said first    communication device being connected to further communication    devices via communication connections of the vectored group, wherein    said vector training signals are transmitted at the same time as    non-payload data carrying signals transmitted on the communication    connections of the vectored group, wherein in time slots between    said vector training signals signals with reduced or no transmit    power are transmitted on the communication connection to be added to    a vectored group, and wherein vectoring is, e.g. vectoring    coefficients are adapted to reduce the influence of crosstalk caused    by signals transmitted from said first communication device to said    second communication device in said communication connections of    said vectored group, and-   after said vector training phase-   an initialization phase, wherein initialization signals are    transmitted on said communication connection to be added to the    vectored group from said first communication device to said second    communication device, and wherein some initialization signals mark    at least some of the positions of the non-payload data carrying    signals of the communication connections of the vectored group at    least during a part of the initialization phase.

However, also other implementations of such an embodiment than thespecific examples described above, in particular embodiments using othercommunication standards, are possible.

The initialization signals may for example be marked at every secondposition of the non-payload data carrying signals or at otherpredetermined positions.

In an embodiment, apart from the positions of the non-payload datacarrying signals also the position of a start of a sequence is marked.In such an embodiment, for example a signal at one or more positionspreceding and/or following the position of the non-payload data carryingsignal may be marked at the beginning of such a sequence. In anotherembodiment, during such a sequence e.g. every second position of thenon-payload data carrying signal may be marked, and at the beginning ofsuch a sequence, a signal (marked or unmarked) at the previous positionof a non-payload data carrying signal may be repeated.

In case of DSL communications, the non-payload data transmitting signalsmay comprise synchronization symbols as explained above.

In case of VDSL2 communication or similar communication standards used,the above-mentioned part of the initialization phase where the markingis performed may comprise one or more of the phases of the groupconsisting of O-P-channel discovery 1, O-P-channel discovery 2,O-P-training 1 and O-P-training 2, although other phases may be used aswell. It should be noted that such methods and corresponding devices maybe used both in cases where all tones or carriers in the training symbollocated at the position of synchronization symbols are marked, e.g.inverted or otherwise modified, and in cases where only some carriers ofthe training symbol located at the position of synchronization symbolsor other predefined symbols are marked.

In addition to the channel discovery of phase 33, in the specialoperation channel in downstream direction at 37 an idle message and at38 a so-called signature message (O-signature) is transmitted. Inconventional DSL systems with this message diverse parameters fortransmission may be communicated to the customer premises equipment. TheO-signature message may for example provide information regarding apower spectral density (PSD), a transmit power level etc. In anembodiment, this message is used to transmit additional informationconcerning the vectoring. For example, in an embodiment the O-signaturemessage may comprise information regarding an offset betweensynchronization symbols between the upstream direction and thedownstream direction, i.e. information indicating if and by which amountthe synchronization symbols transmitted in the upstream direction in thecommunication system are offset from the synchronization symbolstransmitted in the downstream direction, and/or may comprise informationregarding a modulation sequence for the synchronization symbols to beused by the joining line. As indicated previously, for the joining phasea sequence may be reserved for the joining line, for example a sequenceof only +1 or a sequence of alternating +1 and −1, whereas after thejoining each line uses its own specified sequence. The O-signaturemessage may communicate to the customer premises equipment whichsequence is to be used by the particular joining line, for example bycomprising an index of a sequence or the sequence itself.

As mentioned, in the channel discovery phase and possibly in a followingtraining phase the equipment on the side of the customer premises set upand trained its receiver, for example performs a sampling rate recovery,a set up of an automatic gain control, a set up of the symbol timingrecovery, a training of the frequency equalizer etc. Furthermore,through the above-mentioned marking of symbols 90, the customer premisesequipment in the currently discussed embodiment recovers or obtainsinformation regarding the location of the synchronization symbolposition in the downstream direction. As soon as the position of thesynchronization signals in downstream direction has been identified andthe O-signature message of phase 38 has been decoded, the customerpremises equipment in this embodiment can determine the location of thesynchronization symbols in the upstream direction, which may becalculated by the position in the downstream direction and theabove-mentioned offset, if any. Until this time, in the upstreamdirection as indicated by reference numeral 41 only quiet symbols, i.e.only negligible transmit power, are transmitted in the upstreamdirection in the embodiment of FIG. 3. In another embodiment,carrier-pilot signals may be transmitted during this phase. As soon asthe location of the synchronization symbols in downstream direction hasbeen recovered and the O-signature message 38 has been decoded, in phase42 in the upstream direction labeled R-P-Vector Wait synchronizationsymbols are transmitted at the defined upstream synchronization symbolpositions, i.e. synchronously with the synchronization symbolstransmitted on the lines already in the vectored group, and quietsymbols are transmitted between the synchronization signals in theembodiment of FIG. 3. In a different embodiment, carrier-pilot signalsmay be transmitted between the synchronization signals.

In the embodiment of FIG. 3, since only synchronization symbols aretransmitted with quiet symbols in between in phase 42, data transmissionin the upstream direction on the lines already in the vector group isnot or not significantly disturbed. The symbols transmitted during phase42 indicates to the central office that the customer premises equipmentconnected to the joining line has recovered synchronization timing.

In the embodiment of FIG. 3, while the customer premises equipment atthis point has recovered the timing of the synchronization symbols inthe upstream direction and has been assigned a modulation sequence formodulating its synchronization symbols such that their sequence isorthogonal to the sequences transmitted on the lines already in thevectored group, the customer premises equipment at this point does notknow when to start the sequence, i.e. when to use the first element ofthis sequence. In the embodiment of FIG. 3, the start of the sequence istransmitted to the customer premises equipment with a signal labeledO-P-Sync Vec in phase 34 in downstream direction. In a differentembodiment, in phase 33 discussed with reference to FIG. 6 already thestart of the sequence may be transmitted, for example by marking thechannel discovery symbol 19 which corresponds to the start of sequencein a different manner than the other symbols 90.

In the embodiment of FIG. 3, upon receipt of the O-P-Sync Vec signal, inthe upstream direction the phase labeled R-P-Vectoring 1 has starteddenoted by reference numeral 43. In the phase 43, the vectoring istrained to reduce the crosstalk from the joining line to the linesalready in the vectored group in the upstream direction. To this end,during phase 43 in the upstream direction modulated synchronizationsymbols are transmitted according to the modulation sequence assigned tothe respective joining line by O-signature message 38, and quiet symbolsor carrier-pilot symbols are transmitted in between the synchronizationsymbols such that the data transmission on the vectored lines is notdisturbed. During phase 42 and/or 43, the already mentioned reservedWalsh-Hadamard sequence or any other sequence reserved for the joiningline may be used for modulating the synchronization symbols. Thetransmission of the synchronization symbols is in synchronization withthe transmission on the vectored lines. This adaptation will beexplained in some more detail with reference to FIG. 5.

In FIG. 5, some of the components for data transfer in upstreamdirection of the communication system already discussed with referenceto FIG. 4 are shown. On the side of customer premises equipment 84,symbols 50, 52 and 53 are transmitted via the lines, symbol 50 beingtransmitted via the joining line 55 and symbols 52 and 53 beingtransmitted via the vectored lines 56, 57. Again it should be noted thatwhile only two vectored lines are depicted, they are intended torepresent any arbitrary number of vectored lines. The symbols aremodulated onto a plurality of carriers for each line corresponding tothe already mentioned DMT modulation according to the VDSL2 standardused and transferred into time domain signals by inverse fast Fouriertransformers 51, 53 and 54, respectively. The signals are thentransmitted in the upstream direction via the respective lines 55, 56and 57 which are in cable binder 58 to central office 59. Here, thereceived signals are sampled and transferred to the frequency domain viafast Fourier transformers 59, 60 and 61.

A crosstalk canceller 62 is used to cancel crosstalk occurring betweenthe lines in cable binder 58. It should be noted that crosstalkcanceller 62 may fully or partially be implemented using the samecircuit elements as crosstalk precompensator 73, for example by using acommon digital signal processor, but also may be implemented usingseparate elements. Similar to what has already been described for thereceiver part of customer premises equipment 84 with reference to FIG.4, in the receiver part of central office 59 shown in FIG. 5 frequencyequalizers 63, 64 followed by slicers 65, 66 are provided to recoverreceived symbols which in case of error free transmission correspond tosymbols 52, 53 originally sent.

At this stage, i.e. in phase 43 of FIG. 3, no recovery of the symbolssent via joining line 55 is performed in the embodiment of FIG. 3,although in other embodiment symbols sent via the joining line may berecovered as well. Since the synchronization symbols are predeterminedsequences, an error signal e can be calculated describing the differencebetween the sent synchronization symbols and the symbols actuallyreceived for the synchronization symbols. In the embodiment of FIG. 5,with an element 67 the frequency equalization performed in element 63,64 is reversed, although this is not mandatory for obtaining errorsignals. Via one or more feedback lines 68, the error signal is providedto crosstalk canceller 62 which adapts the crosstalk cancellation suchthat the crosstalk, in particular the crosstalk from the joining line tothe vectored lines, is reduced. When this training phase 43 is complete,the first vector training 21 of the embodiment of FIG. 2 is completed,i.e. the vectoring has been adapted to reduce crosstalk from the joininglines to the already vectored lines both in the downstream and in theupstream direction.

The phases specifically representing the first vector training in theembodiment of FIG. 3 are phases 32, 34, 42 and 43. Additionally, inphase 33 in the embodiment of FIG. 3 compared with the channel discoveryaccording to conventional VDSL2 standard symbols as a position ofsynchronization symbols are marked as already explained with referenceto FIG. 6.

Following these phases described above, FIG. 3 shows in the downstreamdirection a phase of channel discovery 1 labeled 35 which is acontinuation of phase 33, i.e. phase 33 and 35 together represent thecomplete channel discovery phase in the downstream direction. Followingphase 35, a synchronization phase 36 labeled O-P-synchro 1 is performedwhich corresponds to a phase defined in the VDSL standard. During thistime, an idle message labeled 39 is transmitted in the special operationchannel. In the upstream direction, following phase 43 the channeldiscovery in the upstream direction denoted with reference numeral 44and labeled R-P-Channel Discovery 1 is performed, which may correspondto the conventional channel discovery. In the special operation channel,at this point also messages may be transmitted, of which in FIG. 3 as anexample an idle message is shown at 55 and a message comprising actualcontent is shown at 46.

It should be noted that the embodiment shown in FIG. 3 merely representsone possibility for implementing the first vector training, and in otherembodiments of the implementation is meant to be used. For example, in adifferent embodiment the O-P-Sync Vec signal labeled 34 in FIG. 3 may betransmitted via a special operation channel. In a different embodiment,such a Sync Vec signal may be transmitted only after the completechannel discovery in the downstream direction, i.e. after phases 33 and35. Still other embodiments are adapted to communication standards otherthan VDSL.

Furthermore, it is to be noted that the features of the embodiments ofFIG. 3, for example the order of downstream and upstream vectoringtraining, may also be used independent of the embodiment shown in FIG.2.

Furthermore, in an embodiment the special operations channel may bemodified compared to the VDSL2 standard. For example, according to theVDSL2 standard, in the special operations channel every tenth carriercarries the same information (for example carriers 11, 21, 31, . . .carry the same information bits). In an embodiment of the presentinvention, during the first vector training, the special operationschannel is modified for example higher modulations, for example 16-QAM,64-QAM, i.e. quadrature amplitude modulation with 16 possibleconstellations or 64 possible constellations. In another embodiment,additionally or alternatively the repetition rate in the frequencydomain of the channels is reduced compared to conventional VDSL2transmission, for example by repeating only every 15^(th) carrier, every20^(th) carrier or every 50^(th) carrier.

In a further embodiment, this increase of bandwidth for the back channelmay also be performed only for the direction from the customer premisesto the central office.

In another embodiment, this variation of the bandwidth of thebackchannel is configured to be variable. For example, the bandwidth ofthe special operation channel for the corresponding phase could beconfigured with messages during the training phase or during any otherphase or point in time preceding the phase where the bandwidth increaseis needed. For example, specific parameters or specific messages couldbe sent via the special operations channel, for example by modifying theso-called “O-TA_Update” message. In an embodiment, this bandwidthincrease is performed at a time where the final power spectral densityand the initial signal to noise ratio estimates of the communicationlines are available, based on which the central office in an embodimentcalculates the bandwidth requirements and communicates to the customerpremises the required parameters like constellation size or channelrepetition mentioned above for the special operation channel.

As already mentioned, in the back channel HDLC may be used which forexample may be formed for each used band.

Next, an example for the second vector training of step 24 of theembodiment of FIG. 2 according to an embodiment of the present inventionwill be discussed with reference to FIG. 7. FIG. 7 schematically showssymbols transmitted via the vectored lines, i.e. the lines already inthe vectored group, and the joining line. Similar to the embodiments ofthe first vector training discussed above, synchronization symbols atthe beginning of each superframe of VDSL transmission are used to adaptthe vectoring. On the vectored lines, synchronization symbols 100 aretransmitted at the beginning of each superframe followed by othersymbols 101 which for example may be payload data symbols. Thesynchronization symbols in the upstream direction are offset by anoffset o from the synchronization symbols in the downstream direction.

On the joining line synchronization symbols 102 are transmitted at thepositions of the synchronization symbols 100 in the already vectoredlines. Also on the joining line, the synchronization symbols 102 in theupstream direction are offset by the offset o from the synchronizationsymbols 102 in the downstream direction. The offset o in an embodimentis communicated to the customer premises equipment of the joining linevia the special operation channel, for example by O-Signature message 38of the embodiment of FIG. 3 as already discussed. Between thesynchronization symbols 102, other symbols 103 are transmitted which maybe any kind of symbols, for example symbols for training the joiningline, data symbols, quiet symbols or pilot symbols. While only signalsfor one vectored line are shown in FIG. 7, synchronization symbols 100are transmitted on all vectored lines synchronously.

As has already been explained above, the synchronization symbols 100,102 on each of the lines are modulated by orthogonal or almostorthogonal sequences in an embodiment, for example Walsh-Hadamardsequences. The sequence to be used on the joining line in an embodimentis also communicated to the corresponding customer premises equipmentvia the special operations channel, for example O-Signature message 38of the embodiment of FIG. 3. Based on the synchronization symbols,similar to what has been discussed with reference to FIGS. 4 and 5 thevectoring, i.e. crosstalk precompensation and crosstalk cancellation,are adjusted to reduce the effect of crosstalk from the vectored linesto the joining line. In this case, error signals generated by therespective receivers in upstream direction and downstream direction ofthe joining line are used. While in an embodiment in the second vectortraining only the crosstalk from the vectored lines to the joining linemay be reduced, in a different embodiment the overall crosstalk may bereduced by taking all error signals, i.e. error signals both from thevectored lines and the joining line, into account. In such anembodiment, for example changes in the crosstalk from the joining lineto the vectored lines caused by changes in the communication between thefirst vector training and the second vector training, for example causedby adjustment during the channel discovery 22 and the training 23 of theembodiment of FIG. 2, may be taken into account.

The above-described embodiments are only to be seen as examples forpracticing the present invention, and modifications and otherembodiments using the same or similar principles are apparent to aperson skilled in the art. As already mentioned, while VDSL2transmission has been taken as a specific example for data transmission,the principles set forth herein may be applied to other kinds ofcommunication including wireless communication. Moreover, while bothupstream and downstream direction have been discussed, the principlesset forth herein may be applied also on unidirectional communicationsystems. Therefore, the above-described embodiments are not to beconstrued as limiting the scope of the present application, and thescope is intended to be limited only by the following claims andequivalents thereof.

What is claimed is:
 1. A method comprising: a first adapting phasewherein vectoring of a vectored group is adapted using non-payload datacarrying signals to reduce the influence of crosstalk induced from acommunication connection to be added to the vectored group to at leastone communication connection of the vectored group, after said firstadapting phase, an xDSL initialization phase wherein data transmissionon said communication connection to be added to the vectored group is atleast partially initialized, after said xDSL initialization phase, asecond adapting phase wherein the vectoring is adapted to reduce theinfluence of crosstalk induced from at least one communicationconnection of the vectored group to the communication connection to beadded to the vectored group, and transmitting payload carrying signalson the communication connections of the vectored group, wherein vectortraining signals are transmitted on the communication connection to beadded to the vectored group at the same time as the non-payload datacarrying signals are transmitted on the communication connections of thevectored group, wherein signals with reduced or no transmit power aretransmitted on the communication connection to be added to the vectoredgroup.
 2. The method of claim 1, wherein said second adapting phasecomprises determining crosstalk coupling parameters indicative ofcrosstalk induced from the at least one communication connection of thevectored group to the communication connection to be added to thevectored group.
 3. The method of claim 1, wherein said signals withreduced transmit power are chosen from the group comprising signals withminimized transmission power and pilot-carrier signals.
 4. The methodaccording to claim 1, wherein said first adapting phase comprisesadapting the vectoring in a downstream communication direction prior toadapting the vectoring in an upstream communication direction.
 5. Themethod of claim 4, further comprising communicating information aboutthe vectoring to a second communication device between said adapting inthe first communication direction and said adapting in the secondcommunication direction, said information about the vectoring comprisingat least one information selected from the group comprising: informationindicating an offset between a position of non-payload data carryingsignals in said first communication direction and a position ofnon-payload data carrying signals in said second communicationdirection; and information indicative of a modulation sequence to beused by said second communication device for modulating non-payload datacarrying signals.
 6. The method of claim 1, comprising a further xDSLinitialization phase wherein data transmission on said communicationconnection to be added to the vectored group is at least partiallyinitialized, said further xDSL initialization phase being executed atleast partially in parallel to said first adapting phase.
 7. The methodof claim 6, wherein said further xDSL initialization phase comprisescommunicating the position of synchronization signals on thecommunication connections of the vectored group to at least onecommunication device coupled to said communication connection to beadded to the vectored group.
 8. A method comprising: transmitting in avector training phase vector training signals from a first communicationdevice to a second communication device on a communication connection tobe added to a vectored group, said first communication device beingconnected to further communication devices via communication connectionsof the vectored group, wherein said vector training signals aretransmitted at the same time as non-payload data carrying signalstransmitted on the communication connections of the vectored group,wherein signals with reduced or no transmit power are transmitted on thecommunication connection to be added to the vectored group, and whereinvectoring is adapted to reduce the influence of crosstalk caused bysignals transmitted from said first communication device to said secondcommunication device in said communication connections of said vectoredgroup, after said vector training phase an xDSL initialization phase,wherein initialization signals are transmitted on said communicationconnection to be added to the vectored group from said firstcommunication device to said second communication device, and whereinthe initialization signals are marked at at least some of the positionsof the non-payload data carrying signals of the communicationconnections of the vectored group at least during a part of the xDSLinitialization phase, and transmitting payload carrying signals on thecommunication connections of the vectored group.
 9. The method accordingto claim 8, wherein said communication connections comprise DSLcommunication lines, and wherein said non-payload data transmittingsignals comprise synchronization symbols.
 10. The method of claim 9,wherein said signals with reduced transmit power comprise signalsselected from the group comprising quiet symbols and pilot-carriersymbols.
 11. The method of claim 8, further comprising: using a firstmodulation for initialization signals not at the positions of saidnon-payload data carrying signals and a second modulation forinitialization signals at positions of said non-payload data carryingsignals to generate said marked initialization signals.
 12. The methodof claim 11, wherein in said second modulation a constellation on atleast one of a plurality of carriers is inverted compared to said firstmodulation.
 13. The method of claim 8, further comprising receiving, atsaid first communication device, information indicating that said secondcommunication device has recovered the position of the non-data carryingsignals.
 14. The method of claim 8, further comprising transmittingvectoring information from said first communication device to saidsecond communication device, said vectoring information comprising atleast one element chosen from the group comprising: informationregarding an offset between transmitted non-payload data carryingsignals in downstream direction and transmitted non-payload datacarrying signals in upstream direction, and information regarding asequence to be used on said communication connection to be added to thevectored group for modulating non-payload data carrying signals.
 15. Themethod of claim 8, further comprising transmitting a start signal fromsaid first communication device to said second communication device,said start signal being indicative of a start of a modulation sequencefor modulating non-payload carrying data.
 16. The method of claim 8,further comprising, after said xDSL initialization phase, a furthervector training phase wherein, further vector training signals arereceived at said first communication device from said secondcommunication device, and the vectoring is adapted to reduce aninfluence of crosstalk caused by signals transmitted from said secondcommunication device to said first communication device to communicationconnections of said vectored group depending on said further vectortraining signals.
 17. An apparatus, comprising: an XDSL receiver, saidreceiver being configured to receive vector training signals transmittedvia a communication connection to be added to a vectored group ofcommunication connections at the same time as non-payload data carryingsignals are transmitted on the communication connections of the vectoredgroup, wherein signals with reduced or no transmit power are transmittedon the communication connection to be added to the vectored group andpayload carrying signals are transmitted on the communicationconnections of the vectored group, said receiver further beingconfigured to receive, after the vector training signals, initializationsignals for initializing data transmission on said communicationconnection to be added to the vectored group, said receivedinitialization signals being marked at positions of said non-payloaddata carrying signals, wherein said apparatus is configured to recoversaid marked positions of said non-payload data carrying signals, saidapparatus further comprising a transmitter configured to synchronizetransmitting of training signals based on said recovered positions ofnon-payload data carrying signals of the communication connections ofthe vectored group.
 18. A system comprising: a first communicationdevice to be coupled to a communication connection to be added to avectored group, at least one second communication device to be coupledto a communication connection of the vectored group, and a vectoringdevice, wherein said system is configured to adapt vectoring of thevectoring device using non-payload data carrying signals to reduce theinfluence of crosstalk from a communication connection to be added tothe vectored group to at least one communication connection of thevectored group in a first adapting phase, to at least partiallyinitialize after said first adapting phase in an xDSL initializationphase data transmission on said communication connection to be added tothe vectored group, and to adapt the vectoring device to reduce theinfluence of crosstalk from the communication connections of thevectored group to the communication connection to be added to thevectored group after said xDSL initialization phase in a second adaptingphase, wherein said first communication device is configured to transmitvector training signals on the communication connection to be added tothe vectored group at the same time as non-payload data carrying signalsare transmitted on the communication connections of the vectored group,wherein signals with reduced or no power are transmitted on thecommunication connection to be added to the vectored group, whereinpayload carrying signals are transmitted on the communicationconnections of the vectored group.
 19. The system of claim 18, whereinsaid first communication device is configured to transmit vectortraining signals on the communication connection to be added to thevectored group at positions of non-payload data carrying signals on thecommunication connections of the vectored group, and wherein signalswith reduced or no power are between said vector training signals duringsaid first adapting phase.
 20. The system of claim 18, wherein saidapparatus being configured to perform a further xDSL initializationphase for at least partially initializing data transmission on saidcommunication connection to be added to the vectored group, said furtherxDSL initialization phase being executed at least partially in parallelto said first adapting phase.
 21. The system of claim 20, wherein saidfirst communication device is adapted to communicate the position ofsynchronization signals on the communication connections of the vectoredgroup to at least one communication apparatus coupled to saidcommunication connection to be added to the vectored group during saidfurther xDSL initialization phase.
 22. A system comprising: a firstcommunication device to be coupled to a communication connection to beadded to a vectored group, at least one second communication device tobe coupled to a communication connection of the vectored group, and avectoring device, said first communication device being configured totransmit vector training signals via the communication connection to beadded to a vectored group, wherein said vector training signals aretransmitted at the same time as non-payload data carrying signalstransmitted by the at least one second communication device on thecommunication connections of the vectored group, wherein signals withreduced or no transmit power are transmitted on the communicationconnection to be added to the vectored group, wherein said vectoringdevice is configured to adapt the vectoring to reduce the influence ofcrosstalk caused by signals transmitted by said first communicationdevice in said communication connections of said vectored group, whereinsaid first communication device is further configured to, after saidvector training phase, transmit initialization signals in an xDSLinitialization phase on said communication connection to be added to thevectored group, and wherein the initialization signals at the positionsof the non-payload data carrying signals of the communicationconnections of the vectored group are marked, wherein payload carryingsignals are transmitted on the communication connections of the vectoredgroup.
 23. The apparatus of claim 22, further configured to provide afirst modulation for initialization signals transmitted not at thepositions of said non-payload data carrying signals and a secondmodulation different from the first modulation for initializationsignals transmitted at positions of said non-payload data carryingsignals.
 24. The apparatus of claim 22, wherein said first communicationdevice is configured to receive vector training signals after said xDSLinitialization phase, and wherein said vectoring device is configured toadapt said vectoring to reduce the influence of crosstalk caused bysignals received by said first communication device to communicationconnections of said vectored group depending on said training signals.